Addition products of alkylene oxides onto alcohols, acids or amines are important industrial products which find frequent use especially as nonionic surfactants.
The alkoxylation is typically performed batchwise, for example in stirred autoclaves or loop reactors at temperatures between 80 and 130° C.; alternatively, the liquid reaction mixture can also be dispersed in an alkylene oxide-containing gas phase. On this subject, reference is made by way of example to a review article in Chem. Res. 25, 9482-9489 (2005). Typically, the compound with the nucleophilic center—for example an alcohol, a carboxylic acid, an ester or an amine—is initially charged together with the catalyst and then the desired amount of alkylene oxide is injected, which, depending on the temperature, generally establishes a pressure of up to 12 bar. Suitable catalysts are basic compounds, for example alkali metal alkoxides, or Lewis acids, the latter having the disadvantage that they tend to form considerable amounts of undesired polyglycol ethers. All these processes involve a liquid reaction mixture and an alkylene oxide-containing gas phase, which is afflicted in generic terms with the problem of insufficient mass transfer at the phases interface and therefore reduced conversion.
The safety problems which additionally arise as a result of alkylene oxides in the gas phase are described, for example, in DE 10054462 B4. The enrichment of ethylene oxide or propylene oxide in the gas phase causes the risk of decomposition. The concentration of the alkylene oxide is therefore kept low by inert gases. According to the teaching of this publication, the alkoxylation is performed in a tubular reactor which is configured as a plate heat exchanger. Alkylene oxide is fed in at several points in the reactor. The amount is selected precisely such that the solubility of the alkylene oxide in the liquid reaction mixture is not exceeded, in order to prevent the local formation of a gas phase.
In this context, reference is also made to the international patent application WO 02/009866 A1 (CPC). This document discloses a microreactor, for purposes including the alkoxylation of fatty alcohols, in which the reaction takes place between two or more essentially plane-parallel plates or layers, at least one of these plates or layers being a fluid conduction plate and being structured and/or arranged such that the fluid reactant flows only owing to the influence of gravity and/or to capillary forces within at least one essentially uninterrupted capillary thread along the surface or layer, and as it does so comes into contact with the gaseous reactant and reacts. However, this is nothing more than a miniaturized falling-film reactor in which the reaction takes place at the interface of liquid and gaseous phases, and which is therefore afflicted with the technical disadvantages already described above.
It was therefore an object of the present invention to provide an improved process for preparing alkylene oxide addition products, which enables more intense mass transfer and hence higher conversions, while also permitting higher alkylene oxide concentrations, without this being at the cost of reduced occupational safety.